There are daily charter flights between the US and Cuba... including American, United, Continental etc... so there is absolutely no reason Air Transat would not be allowed to land its Cuba flight in the US under normal circums let alone if they declared an emergency... fact is that Air Transat released the following this afternoon :
MONTREAL, March 7 - Air Transat wishes to point out that the decision to return to Varadero rather then land in Florida was made by the captain together with the Operational Control Center because the Company has access to maintenance staff at this airport. It is untrue that American authorities were opposed to allowing the plane to land on their territory.